Dyes for cotton and viscose

Direct dyes are so designated because cotton and cellulosic fibres are readily dyed in a weak aqueous solution of the dye, containing salt or sodium sulphate, kept near boiling point. Prior to the introduction of Congo Red the 

The triazinyl ring acts as a chromophoric block, isolating the A/Q chromogen from the azo system and resulting in a clear green by a simple 

An important set of diazo direct dyes are made by two routes  

It will be seen that all the direct dyes so far discussed are extended, linear molecules—a characteristic of the class. It has been shown that the distance between the two azo groups in a substantive dye is 1.08 nm and that of the polymer unit length in cellulose is 1.03 nm, so that the operation of van der Waals forces is facilitated, though there is clear proof that this is not the only factor present since substantivity disappears in CA and CT fibres. H-bonding is invoked as a source of attachment, the hydrogen atoms of—OH groups in the cellulose unit bonding with N of an azo link 

Additionally H-bonds can in theory be set up between the oxygen atom of cellulosic-OH and the H atoms in amino and carbamido groups in the dye 

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Uses Dye for cotton and viscose rayon analytical reagent in detection of aluminum, magnesium, mercury, silver, uranium biological stain pH indicator (violet 1.2 to red 4.0)... 

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